2 solutions

Solution 1

Create an Setup and Deployment project on your development machine - that should automatically include and register any necessary DLL files (which are probably on your dev machine anyway, but may need configuration on the target.

Just copying the debug folder is a poor idea anyway, as debug EXEs contain information you may not want to distribute!

Solution 2

Copy the required DLLs from <QtDir>/bin on your development machine to the installation folder on the client machine (the folder where the executable is stored)

If the executable is build using a Microsoft compiler you might also need to install the redistributable files for that compiler on the client machine (run vcredist_x86.exe for 32-bit Windows or vcredist_x64.exe for 64-bit Windows)

If the executable is build using the MinGW compiler you must install MinGW on your development machine (can be done using the Qt online installers), then you copy the required DLLs from <MinGWDir>/bin to the installation folder on the client machine.

Note: When distributing executables, it is recommended to distribute the release version and not the debug version.